STUDIES ON THE ELECTROCHEMICAL BEHAVIOUR OF SOME METALLIC ' . ALLOYS IN AQUEOUS SOLUTIONS

Abstract

The work in this thesis is aimed at studying the electrochemical and corrosion behaviour of some metals and. alloys in aqueos media: • The pitting corrosion behaviour of zinc in each of the following solutions NaCl, NaBr and Nai was studied by potentiodynamic and cyclic voltammetry techniques under the influence of different experimt:ntal variables. In these solutions, the anodic sweep exhibits an active passive transition prior to the initiation of pitting corrosion. The active dissolution region displays one cathodic peak A1 . Passivity is due to the existence of a protective film of ZnO on the anodic surface. At a certain specific potential, pitting potential, breakdown of the anodic passivity occurs caused by field-stimulating halide penetration into the passive oxide film at point defects. The aggressiveness of the halide ions towards the stability of the passive film decreases in the order: Cl' > Br• > r. The susceptibility of zinc to pitting corrosion enhances with

increasing the halide ion concentration and the temperature but decreases ; with the sweep rate. The cathodic sweep shows two reduction peaks C1

and C2 . The more negative cathodic peak C1is a conjugate of the anodic peak A1 while the cathodic peak C2 is related to the electroreductionof the pitting corrosion products. The existence of pitting corrosion was confirmed by scanning electron microscopy.